Method for producing Phillips catalysts for polymerizing olefins with better productivity rates in the particle-form process

ABSTRACT

In a process for producing Phillips catalysts in which an oxidic support material is treated in suspension with a chromium salt solution and subsequently, after removing the solvent, calcined in an oxygen-containing atmosphere at above 300° C., the oxidic support material and/or the catalyst after calcination are/is, according to the present invention, comminuted until a mean particle size of &lt;100 mum has been reached and the proportion of particles having a size of &lt;50 mum is at least 30%, preferably in the range from 40 to 80%. A process for preparing homopolymers or copolymers of ethene in a loop reactor at from 30 to 150° C. under a pressure in the range from 0.2 to 15 MPa in the presence of a catalyst produced by the process of the present invention is also provided.

The present invention relates to a process for producing Phillipscatalysts in which an oxidic support material is treated in suspensionwith a chromium salt solution and subsequently, after removing thesolvent, calcined in an oxygen-containing atmosphere at above 300° C.

A process of this type is known and is comprehensively described in, forexample, DE-A 25 40 279. The catalysts produced as described there arealso comminuted and have particle sizes in the range from 20 to 2,000μm, in particular from 40 to 300 μm.

DE-A 36 40 802 and DE-A 36 40 803 state that restricting the oxidicsupport to a particular, very narrow particle size distribution in therange from 50 to 150 μm gives chromium trioxide catalysts which givepolymers having an improved particle morphology at equal or highercatalyst productivity.

Finally, it has been found in U.S. Pat. No. 5,641,842 that Phillipscatalysts having particle sizes of >75 μm are advantageous for themorphology of polyethylene prepared therewith.

After evaluation of the relevant literature, it can be said in summarythat classification of the oxidic support material influences thecatalyst productivity and the polyethylene morphology. According to theliterature, the best results are generally achieved using relativelycoarse catalysts, i.e. those having particle sizes of >50 μm.

It is an object of the present invention to provide a new process whichmakes it possible to produce Phillips catalysts which further increasethe productivity of the polymerization of ethylene in loop precipitationprocesses and, in particular, allow increased polyethylene solidscontents within the polymerization reactor.

We have found that this object is achieved by a process of the generictype mentioned at the outset, whose defining features are that theoxidic support material before calcination and/or the catalyst aftercalcination are/is comminuted until a mean particle size of <100 μm hasbeen reached and the proportion of particles having a size of <50 μm isat least 30%.

According to the present invention, the oxidic support material used isa silica gel which has a solids content, calculated as silicon dioxide,in the range from 10 to 30% by weight, preferably from 11 to 25% byweight, and is largely spherical. Such a silica gel is obtained byintroducing a solution comprising sodium water glass or potassium waterglass into a twisting stream of a mineral acid longitudinally andtangentially to the flow direction of the stream and spraying thesilicic acid hydrosol formed into a gaseous medium so as to formdroplets. The sprayed hydrosol then solidifies in the gaseous medium toform spherical particles and is freed of adhering salts by washing withwater.

The spherical hydrosol is then treated with an organic liquid selectedfrom among alcohols having from 1 to 4 carbon atoms until at least 60%of the water present in the hydrosol has been extracted. The dewateredhydrogel which has been treated with the alcoholic liquid is then drieduntil at >160° C. using an inert carrier gas the residual alcoholcontent is less than 10% by weight.

The xerogel obtained in this way is then loaded with chromium from a0.05-5% strength by weight solution of chromium trioxide in a ketonehaving from 3 to 5 carbon atoms or from a 0.05-15% strength by weightsolution of a chromium compound which is converted into chromiumtrioxide under the conditions of the calcination in an alcohol havingfrom 1 to 4 carbon atoms and the solvent is subsequently evaporatedunder reduced pressure.

For the calcination, the chromium-laden oxidic support material ismaintained at from 300 to 1,100° C. in a water-free, oxygen-containinggas stream for from 10 to 1,000 minutes.

The comminution according to the present invention of the oxidic supportmaterial or of the catalyst material obtained as described above iscarried out by dry milling using a ball mill or in a beater mill asdescribed, for example, in DE-A 36 40 802. The milling time necessary toachieve the desired particle size is determined by taking samples atparticular time intervals.

In the olefin polymerization in which the catalyst produced according tothe present invention is used, it is possible to prepare homopolymers ofethylene or copolymers of ethylene with a comonomer having from 3 to 12carbon atoms in an amount of up to 10% by weight of comonomer. Thepolymerization itself is carried out at from 30 to 150° C. under apressure in the range from 0.2 to 15 MPa.

It has surprisingly been found that, at a constant reactor output, thePhillips catalysts having a particle size of <100 μm used according tothe present invention result in an increase in the average residencetime of the catalyst in the reactor and that the catalyst productivityincreases at the same time. The higher catalyst productivity presumablyresults from significantly higher polyethylene solids concentrationsbeing able to be achieved in the loop precipitation process,particularly in loop reactors, when using the catalysts producedaccording to the present invention than when using the catalystsdescribed in the literature, which customarily have particle sizesof >100 μm. Furthermore, it has surprisingly been found that thePhillips catalysts produced according to the present invention make itpossible to achieve comparable results in terms of the morphology of thepolyethylene prepared therewith to those obtained using the conventionalcatalysts having particle sizes of >100 μm.

The process of the present invention gives particularly optimal resultswhen the proportion of particles having a size of <50 μm is in the rangefrom 40 to 80%.

For the purposes of the present invention, all particle size data weredetermined in accordance with DIN 53 477, sieve analysis.

The examples and comparative examples described below show that in thecase of the catalysts described in the literature formation of depositson the reactor wall occurs even at relatively low PE solids contentsabove 40% by weight, while the catalysts produced according to thepresent invention allow solids contents of about 60% without reactorfouling in loop reactors.

Production of the oxidic support material:

EXAMPLE 1 (according to the present invention)

The catalyst support was produced as described in DE-A 36 40 802, exceptthat the dried xerogel spheres were milled by means of a beater mill toa mean particle size in the range from 1 to 100 μm and were sieved sothat the proportion of particles having a size of <50 μm was 80%.

EXAMPLE 2 (according to the present invention)

The catalyst support was produced as described in Example 1, except thatthe proportion of particles having a size of <50 μm was only 30%.

EXAMPLE 3 (comparative example)

The catalyst support was produced as described in Example 1, except thatthe proportion of particles having a size of <50 μm was only 15%.

EXAMPLE 4 (comparative example)

The catalyst was produced as described in Example 1 of DE-A 36 40 802(page 7, line 15). The support particles had a mean particle size in therange from 50 to 100 μm (page 8, line 26).

EXAMPLE 5 (comparative example)

The catalyst was produced as described in Comparative Experiment 1 inDE-A 36 40 802 (page 9, line 16). The support particles had a meanparticle size in the range from 1 to 300 μm.

Production of catalysts 1-4:

Catalyst 1

Catalyst 1 was produced as described in Example 1 of DE-A 36 40 802(page 8, line 31), except that 15 kg of the oxidic support material fromExample 1 were used.

Catalyst 2

Catalyst 2 was produced as described in Example 1 of DE-A 36 40 802(page 8, line 31), except that 15 kg of the oxidic support material fromExample 2 were used.

Comparative catalyst 3

Comparative catalyst 3 was produced as described in Example 1 of DE-A 3640 802 (page 8, line 31), except that 15 kg of the oxidic supportmaterial from Example 3 were used.

Comparative catalyst 4

Comparative catalyst 4 was produced as described in Example 1 of DE-A 3640 802 (page 8, line 31) using 15 kg of the oxidic support material fromExample 4, as described in DE-A 36 40 802.

Comparative catalyst 5

Comparative catalyst 5 was produced as described in Example 1 of DE-A 3640 802 (page 8, line 31) using 15 kg of the oxidic support material fromExample 5, as described in DE-A 36 40 802.

Polymerization:

For the polymerization of ethylene, use was made of a customary andknown loop reactor whose reaction space consisted of a tube circuithaving a capacity of 6 m³. At a pressure of 4.0 MPa, the reaction spacecontained a suspension comprising liquid i-butane, solid polyethylene,6% by weight of dissolved ethene and 0.4% by weight of dissolved1-hexene. The polymerization temperature was from 103.5 to 103.8° C.

The suspension was pumped around the reactor by means of a propellerpump operating at 3,000 rpm. At a constant reactor output of 900 kg ofPE/h, attempts were made to increase the polyethylene solids content inthe suspension as far as possible. The achievable PE solids contentswere limited by occurrence of reactor fouling (formation of deposits onthe reactor walls or increases in the power drawn by the propellerpump).

The following table shows the results achieved using the catalysts 1 to5:

Cat. 1 Cat. 2 Cat. 3 Cat. 4 Cat. 5 Max. PE solids 58 56 40 38 39 contentin the reactor (% by weight) Catalyst 12,450 11,750 6950 7200 6650productivity (kg of PE/kg of cat.) Melt index HLMI 6.2 6.4 6.5 6.2 6.4190° C./21.6 kp in accordance with DIN 53735 (g/10 min) Bulk density in500 500 490 500 480 accordance with DIN 53468 (g/l) Sieve analysis inaccordance with DIN 53457  <125 μm (%) 0.5 0.6 0.4 0.4 0.6 >2000 μm (%)0.2 0.3 0.2 0 0.5

Significantly higher PE solids contents in the loop reactor can beachieved when using the catalysts produced according to the invention,as a result of which the catalyst productivities increase significantly.In the cases of comparative catalysts 3 to 5, attempts to increase thePE solids contents to more than 40% by weight failed due to reactorfouling.

The bulk density of the polyethylene and the proportion of fumes and ofcoarse material <125 μm, >2000 μm) are virtually unchanged in theexamples according to the present invention and are at the same level asthose in the comparative examples.

We claim:
 1. A process for producing Phillips catalysts in which anoxidic support material is treated in suspension with a chromium saltsolution or a solution of chromium trioxide and subsequently, afterremoving the solvent, calcined in an oxygen-containing atmosphere atabove 300° C., wherein the oxidic support material before calcinationand/or the catalyst after calcination are/is comminuted until a meanparticle size of <100 μm has been reached and the proportion ofparticles having a size <50 μm is at least 30% but not more than 80%. 2.A process as claimed in claim 1, wherein the oxidic support materialused is a silica gel which has a solids content, calculated as silicondioxide, in the range from 10 to 30% by weight, and is largelyspherical.
 3. A process as claimed in claim 2, wherein the sphericalsilica gel is prepared from spherical hydrosol which is treated with anorganic liquid selected from among alcohols having from 1 to 4 carbonatoms until at least 60% of the water present in the hydrosol has beenextracted, and the resulting dewatered hydrogel is then dried at >160°C. using an inert carrier gas until the residual alcohol content is lessthan 10% by weight to form the spherical silica gel.
 4. A process asclaimed in claim 3, wherein the dried silica gel is loaded with chromiumfrom a 0.05-5% strength by weight solution of chromium trioxide in aketone having from 3 to 5 carbon atoms or from a 0.05-15% strength byweight solution of a chromium salt compound which is converted intochromium trioxide under the conditions of the calcination in an alcoholhaving from 1 to 4 carbon atoms and the solvent is subsequentlyevaporated under reduced pressure.
 5. A process as claimed in claim 1,wherein the calcination of the chromium-laden oxidic support material iscarried out in a water-free gas stream containing at least 10% by volumeof oxygen for from 10 to 1,000 min at from 300 to 1,100° C.
 6. A processas claimed in claim 1, wherein the comminution of the oxidic supportmaterial and/or of the catalyst material obtained after calcination iscarried out by dry milling using a ball mill or in a beater mill.
 7. Aprocess as claimed in claim 1, wherein comminution is continued untilthe proportion of particles having a size of <50 μm is in the range from40 to 80%.
 8. A process for olefin polymerization in which homopolymersof ethylene or copolymers of ethylene and a comonomer having from 3 to12 carbon atoms in an amount of up to 10% by weight of comonomer areprepared, wherein the polymerization is carried out in the presence of aPhillips catalyst produced by a process as claimed in claim 1 at from 30to 150° C. under a pressure in the range from 0.2 to 15 MPa.
 9. Aprocess as claimed in claim 8, wherein the polymerization is carried outas a precipitation polymerization in a loop reactor.